spring in action 学习笔记八：用@Primary 或者@Qualifier消除@Autowired引起的歧义现象

Posted 2020-09-22 技术让世界更精彩

　　首先解释一下@Primary和@Qualifier这两个注解的意思：@Primary的意思是在众多相同的bean中，优先使用用@Primary注解的bean.而@Qualifier这个注解则指定某个bean有没有资格进行注入。

示例代码的思路是：1.一个接口Dessert和这个接口的三个实现类，2.再在一个类（AbrahamLincoln）中自动注入Dessert这个接口，3.用自动扫描机制自动创建bean.

如果不用@Primary和@Qualifier注解，势必出现如下错误：NoUniqueBeanDefinitionException.

 

　　示例代码如下：【用@Primary来解决问题】

Dessert接口的代码如下：

 1 package com.advancedWiring.ambiguityInAutowiring;
 2 
 3 import org.springframework.stereotype.Component;
 4 
 5 /**
 6  * Created by ${秦林森} on 2017/6/9.
 7  */
 8 @Component
 9 public interface Dessert {
10     void amI();
11 }

Cake的代码如下：

 1 package com.advancedWiring.ambiguityInAutowiring;
 2 
 3 import org.springframework.beans.factory.annotation.Qualifier;
 4 import org.springframework.context.annotation.Primary;
 5 import org.springframework.stereotype.Component;
 6 
 7 /**
 8  * Created by ${秦林森} on 2017/6/9.
 9  */
10 @Component
11 public class Cake implements Dessert {
12     @Override
13     public void amI() {
14         System.out.println("I am cake");
15     }
16 }

Cookie的代码如下：

package com.advancedWiring.ambiguityInAutowiring;

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Component;


/**
 * Created by ${秦林森} on 2017/6/9.
 */
@Component
public class Cookie implements Dessert {
    @Override
    public void amI() {
        System.out.println("I am cookie");
    }
}

IceCream的代码如下：

package com.advancedWiring.ambiguityInAutowiring;

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Component;


/**
 * Created by ${秦林森} on 2017/6/9.
 */
@Component
@Primary//注意这里用了@Primary这个注解。
public class IceCream implements  Dessert{
    @Override
    public void amI() {
        System.out.println("I am ice cream");
    }
}

DessertConfig的代码如下：

 1 package com.advancedWiring.ambiguityInAutowiring;
 2 
 3 import org.springframework.context.annotation.ComponentScan;
 4 import org.springframework.context.annotation.Configuration;
 5 
 6 /**
 7  * Created by ${秦林森} on 2017/6/9.
 8  */
 9 @Configuration
10 /**
11  * 用@ComponentScan自动扫描创建bean
12  */
13 @ComponentScan(basePackageClasses = Dessert.class)
14 public class DessertConfig {
15 
16 }

测试类的代码如下：

 1 package com.advancedWiring.ambiguityInAutowiring;
 2 
 3 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 4 
 5 /**
 6  * Created by ${秦林森} on 2017/6/9.
 7  */
 8 public class AmbiguityTest {
 9     public static void main(String[] args) {
10         AnnotationConfigApplicationContext ac = new AnnotationConfigApplicationContext(DessertConfig.class);
11         Dessert dessert = ac.getBean(Dessert.class);
12         dessert.amI();
13     }
14 }

 二：用@Qualifier这个注解来解决问题：

核心代码如下：

 1 package com.advancedWiring.ambiguityInAutowiring;
 2 
 3 import org.springframework.beans.factory.annotation.Qualifier;
 4 import org.springframework.context.annotation.Primary;
 5 import org.springframework.stereotype.Component;
 6 
 7 
 8 /**
 9  * Created by ${秦林森} on 2017/6/9.
10  */
11 @Component
12 @Qualifier("crispy")
13 public class Cookie implements Dessert {
14     @Override
15     public void amI() {
16         System.out.println("I am cookie");
17     }
18 }

 1 package com.advancedWiring.ambiguityInAutowiring;
 2 
 3 import org.springframework.beans.factory.annotation.Autowired;
 4 import org.springframework.beans.factory.annotation.Qualifier;
 5 import org.springframework.stereotype.Component;
 6 
 7 /**
 8  * Created by ${秦林森} on 2017/6/9.
 9  */
10 @Component
11 public class AbrahamLincoln {
12     private Dessert dessert;
13 
14     public Dessert getDessert() {
15         return dessert;
16     }
17     @Autowired
18     @Qualifier("crispy")
19     public void setDessert(Dessert dessert) {
20         this.dessert = dessert;
21     }
22 }